Quite often solid precursors are used for vapor reactants, because liquid or gaseous precursors for a certain element may not be readily available or do not exist at all. Such solid precursors are useful in a variety of contexts, including, without limitation, atomic layer deposition (ALD) and other semiconductor fabrication processes. However, it is more difficult to use solid precursors than liquid and gaseous precursors.
Basically, the handling of solid precursors seems to be straightforward. Solid precursor is loaded into a container that is heated to a sufficiently high temperature. The precursor sublimes and the precursor vapor is conducted to a reaction space where it is used for the deposition of thin film on the substrate surface.
Precursor powder generally has rather poor thermal conductivity. The thermal conductivity of the precursor bulk may be low and/or there may be empty voids between the precursor particles with little contact surface between the particles, which is undesirable for the conduction of heat energy through the precursor. The volume of the voids depends on the packing density of the precursor powder. At low pressures, heat transport by convection is also generally inefficient, especially when the precursor volume consists of very small voids between the precursor particles. Heat transport by radiation is also generally inefficient because the temperature differences are relatively small and the radiation view factor (line-of-sight paths available for radiant heating) for the bulk of the powder is essentially zero.
When the precursor vessel is heated from outside, the precursor may have a sufficiently high temperature near the vessel walls while the center parts of the precursor powder are insufficiently heated. This temperature differential results from the long period of time required to heat the centrally located portions of the precursor powder in the precursor vessel. In addition, sublimation of the non-centrally located precursor consumes thermal energy, further contributing to the center of the precursor powder volume remaining at a lower temperature than the powder proximate the vessel surfaces throughout the process. During ALD pulsing, this temperature differential can cause the solid source to demonstrate a poor recovery rate after using the precursor source for an extended period, because it becomes more and more difficult to reach an equilibrium state in the gas phase of the precursor vessel. Although ALD processes are relatively insensitive to small drifts in pulse concentration, significant decreases in the recovery rate can cause problems, such as less than full surface coverage of a semiconductor wafer (or other substrate) with the precursor molecules.
Temperature differences inside the precursor vessel lead to the sublimation of the precursor into the gas phase in hotter parts of the vessel volume and to the condensation of the precursor back to the solid phase in cooler parts of the vessel volume. Often the top surface of the precursor seems to be cooler than the rest of the precursor. It has been observed that a hard and dense crust forms over the surface of the heated precursor over time, causing a pulse concentration drift in the process employing the vapor reactant (e.g. ALD). The crust limits the diffusion of precursor molecules from the bulk material to the surface and eventually into the gas phase. The result is a decrease in the observed sublimation rate of the precursor. Initially, the solid precursor source works well but later it is difficult to get a high enough flux of precursor molecules from the source into the reaction chamber, despite the fact that a significant amount of solid precursor remains in the precursor vessel.
Another consideration in sublimation vessel design is that prolonged presence of heated corrosive precursors places heavy demands on those materials in contact with precursors in the precursor vessel.
The preferred embodiments of the invention provide means for improving the uniformity of the source temperature in the whole solid precursor vessel volume. In accordance with one aspect of the present invention, inert materials that have high thermal conductivity are mixed with the solid precursor to improve the thermal conductivity through the precursor. For example, the inert materials can comprise particles, fibers, rods, or other elements with high thermal conductivity distributed through the precursor vessel and intermixed with precursor powder.
In accordance with one embodiment of the invention, a method of producing a vapor from a solid precursor for processing a substrate is provided, including placing solid units of precursor into a vessel and interspersing a thermally conductive material through the precursor. The thermally conductive material thereby preferably serves to conduct heat energy throughout the units of precursor. A vapor is then formed through applying heat energy to both the thermally conductive material and the solid units of precursor. In one embodiment, after vapor formation, the vapor is routed from the vessel to a reaction chamber and reacted to deposit a layer on a substrate.
In accordance with another embodiment, a substrate processing system is provided for forming a vapor from a solid precursor by distributing heat throughout the precursor. The provided system comprises a heat conducting vessel configured to hold units of solid precursor, thermally conductive elements being interspersed with the units of solid precursor. A heater is also provided for heating both the precursor and the thermally conductive elements.
In accordance with yet another embodiment, a substrate processing system for forming a vapor from a solid precursor is provided. The system includes a vessel configured to hold units of solid precursor and a microwave generator adjacent to the vessel. The generator is configured to transmit heat energy in the form of microwave energy to effectuate the heating of the precursor.
In accordance with a further embodiment, a mixture for producing a vapor used in substrate processing is provided. The mixture includes a batch of precursor for producing a substrate processing vapor and a plurality of heat transmitting solid forms interspersed through the batch of precursor. The plurality of heat transmitting solid forms collectively increase the thermal conductivity of the batch of precursor.
Advantageously, implementation of the preferred embodiments decreases crust formation at the precursor surface and enhances the sublimation of the precursor. In addition, improving sublimation rate uniformity over the operational life of the precursor batch decreases the amount of unused precursor. Refilling of the precursor vessel is also needed less often due to more efficient material utilization. Another benefit of the present invention is the improvement of the thin film thickness uniformity on substrates by processes employing vapor from the solid precursor by encouraging rapid recovery of the partial pressure of reactant in the gas phase of the vessel to a steady-state value (one such value is P0, the saturation vapor pressure of the material) from pulse-to-pulse.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.